What happened
On 7 February 2018, a scheduled passenger flight operated by Singapore Airlines, involving an Airbus A330-300 with registration 9V-SSE, was en route from Dhaka, Bangladesh, to Singapore. While climbing through 30,000 feet within Myanmar airspace, the flight crew experienced a loud bang accompanied by intense airframe vibrations.
Following standard operating procedures, the pilot-in-command took control of the aircraft and initiated the shutdown of the No. 1 engine. Despite the engine shutdown, the vibrations persisted. The crew declared a PAN urgency call to Yangon Air Traffic Control and diverted the aircraft to Mandalay International Airport. The aircraft landed safely without any injuries to the 203 persons on board. Upon landing, ground services identified an oil leak from the left engine.
The investigation
An investigation by the TSIB examined the damage to the Rolls Royce Trent 700 engine and the mechanics of the failure. Post-flight inspections revealed that the fan blades and the fan case had sustained significant damage. Specifically, one fan blade had broken, with over 75% of its material missing. The released fragment had penetrated the fan case but was caught by the internal Kevlar layers.
Investigators also found that the Low Pressure (LP) shaft had experienced heavy rubbing against the Intermediate Pressure (IP) shaft, leading to deformation and cracking of the shaft. Additionally, the vibration caused the fuel pipes at the inlet and outlet of the LP fuel pump to fracture.
Technical analysis of the broken blade segment revealed that the failure was caused by a fatigue crack originating at the inner convex surface bond. While an ultrasonic C-Scan inspection had been performed on this blade in 2016, the defect was too small to meet the rejection threshold at that time.
Findings
- The primary cause of the incident was a fatigue failure of a fan blade.
- The engine's fuse system, designed to mitigate imbalance during a fan blade off event, operated, but the second fuse activated with a delay because the released blade section was only a partial fragment rather than a full blade.
- This delay in the fuse system allowed for a brief period of contact between the LP and IP shafts, resulting in shaft deformation.
- The C-Scan inspection method used during overhaul failed to reject the blade because the detected defect was below the established threshold.
- The vibration from the event caused the cracking of the LP fuel pump pipes, which presented a potential safety hazard due to the release of fuel.
Safety action
Following the investigation, several safety improvements were initiated:
- The engine manufacturer issued a service bulletin to accelerate the mandatory inspection interval for fan blades from 2,400 to 1,200 flight cycles.
- New program logic was developed to ensure the engine shuts down more rapidly during a fan blade off event to protect the LP shaft.
- The manufacturer began reviewing fan blade designs to reduce stress and is working on enhancing the C-Scan process to detect smaller, critical defects.